Math.easeInOutQuad = function (t, b, c, d) {
    t /= d / 2;
    if (t < 1) {
        return c / 2 * t * t + b;
    }
    t--;
    return -c / 2 * (t * (t - 2) - 1) + b;
};

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
var requestAnimFrame = (function () {
    return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function (callback) { window.setTimeout(callback, 1000 / 60); };
})();

// because it's so fucking difficult to detect the scrolling element, just move them all
function move (amount) {
    document.documentElement.scrollTop = amount;
    document.body.parentNode.scrollTop = amount;
    document.body.scrollTop = amount;
}

function position () {
    return document.documentElement.scrollTop || document.body.parentNode.scrollTop || document.body.scrollTop;
}

export function scrollTo (to, duration, callback) {
    const start = position();
    const change = to - start;
    const increment = 20;
    let currentTime = 0;
    duration = (typeof (duration) === 'undefined') ? 500 : duration;
    var animateScroll = function () {
        // increment the time
        currentTime += increment;
        // find the value with the quadratic in-out easing function
        var val = Math.easeInOutQuad(currentTime, start, change, duration);
        // move the document.body
        move(val);
        // do the animation unless its over
        if (currentTime < duration) {
            requestAnimFrame(animateScroll);
        } else {
            if (callback && typeof (callback) === 'function') {
                // the animation is done so lets callback
                callback();
            }
        }
    };
    animateScroll();
}
